CS 130a DISCUSSION SESSION 2

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CS 130a - DISCUSSION SESSION 2
Shravan Samindla
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LZW Its the Code

• Compression
• Given a text, at each step
• generate code (consult table)?
• update code table
• are
• Decompression
• Given a code, at each step
• reconstruct the code
• reconstruct the text
• are

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LZW Compression, Decompression

• a b b a a b b a a b b a b b a

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LZW Compression Example 1

• a b b a a b b a a b b a b b a

input text
0 1 1 0 2 4 6 8
coded text
ab 2
bb 3
ba 4
code table
aa 5
abb 6
baa 7
abba 8

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LZW Decompression Example 1

• 0 1 1 0 2 4 6 8

coded text
a b b a ab ba abb abba
coded text
2 ab
3 bb
4 ba
code table
5 aa
6 abb
7 baa
8 abba

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LZW Compression Example 2

• a a a b b b a a a a b b a a a

input text
0 2 1 4 2 3 1 6
coded text
aa 2
aab 3
bb 4
bba 5
code table
aaa 6
aabb 7
ba 8

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LZW Decompression Example 2

• 0 2 1 4 2 3 1 6

coded text
a aa b bb aa aab b aaa
coded text
2 aa
3 aab
4 bb
code table
5 bba
6 aaa
7 aabb
8 ba

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Try this...

• code using LZW
• a b b a b a a b b a b b a b a b b a

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Whats wrong?

• 0 2 3 3 2 2
• 0 2 4 2 1 3

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Priority Queues - Heaps

• Priority queue
• insert
• deleteMin
• Heaps
• One way of achieving priority queues
• Why not use linked lists?

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Heaps (contd...)?

• Heaps
• binary heaps
• complete binary tree (so the height is...)?
• heap order property (what do we gain?)?
• In minheap,
• val(parent)
• val (parent)
• So, always make sure that heap order property
  holds

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Why is buildheap linear?
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insert

• O(log N)?

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delete

• O(logN)?

the thing to remember while deleting from an
arbitrary location, we can move one of two ways
up or down
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Heaps - Operations

• decreasekey (p,?)?
• sift-up
• increasekey (p,?)?
• sift-down
• remove (p)?
• ??

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Is the heap representation unique?

• because...
• is complete ordering enforced?

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THANK YOU